Implements a stale-while-revalidate strategy for the SubscriptionsManager.
- Loads subscriptions from localStorage immediately to show UI state.
- Fetches fresh data from relays in the background.
- Preserves cached data if relays fail or return empty results (preventing "No subscriptions found" errors).
- Updates cache optimistically on subscribe/unsubscribe actions.
- Modified `tests/subscriptions-manager.test.mjs` to remove the assertion for legacy Kind 30002 support.
- Updated the test to verify that only the current `SUBSCRIPTION_SET_KIND` (30000) is queried.
- Fixed JSDOM `navigator` mocking in the test file to resolve `TypeError: Cannot set property navigator...` errors.
- Verified all tests pass with the new implementation.
google-labs-jules[bot]